This application is based on Japanese Patent Application Nos. 11-119386 (1999) filed Apr. 27, 1999 and 11-139529 (1999) filed May 20, 1999 in Japan, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to an optical switch for an optical add drop multiplexer (OADM), an optical cross-connection (OXC) or an auxiliary system switching. The optical switch is used for light path routing/switching in a subscriber optical network such as a fiber to the desk (FTTD) or an optical local area network (LAN), and in an infrastructure optical network system such as optical interconnections in a communication processing apparatus.
Further, the present invention relates to an optical switch that is polarization insensitive within an optical communication wavelength bandwidth between 1.3 and 1.65 xcexcm and an optical part comprising this optical switch.
2. Description of the Related Art
In these days of the advent of multimedia communication, much attention is being paid to a wavelength division multiplexing (WDM) type network in which all relevant operations such as switching and routing are performed using light, in order to accommodate a future increase in demands for transmission capacity. A key device for the WDM type network is an optical switch for the OADM or an optical switch for the OXC. The optical switch is an essential device providing flexibility and reliability for a subscriber optical network such as a FTTD or a LAN, and an infrastructure optical network system such as optical interconnections in a communication processing apparatus.
In a view of both good connectivity with optical fibers and mass production, thermo-optics (TO) switches using silica-based planar lightwave circuits (PLC) technology are generally used as optical switch for OADM or OXC. These TO switches are capable of switching in several microseconds. However, they have various problems; that is, they must be continuously supplied with an electric voltage in order to maintain the ON switching state (they have no self-latching function). Both their extinction ratio ( greater than 35 dB) and their crosstalk ( less than xe2x88x9235 dB) are not enough for the optical network systems, and their size are also large.
In contrast, mechanical switches are superior to the TO switch in the point of that they have the self-latching function and can easily achieve a high extinction ratio ( greater than 50 dB) and a low crosstalk ( less than xe2x88x9250 dB).
By way of examples, a mechanical switch has been proposed, which can switch a light path because mercury sealed in its slit is moved by powering up micro thin-film electrodes provided near a slit formed at a crossing point of optical waveguides crossing each other in an optical waveguide layer (Makoto Sato, xe2x80x9cElectro capillarity Optical Switch,xe2x80x9d IEICE TRANS. COMMUN., VOL. E77-B, No. 2, PP. 197-203, FEBRUARY 1994). In addition, similar to the mechanical switch, an optical switch has been proposed, which can switch the light path because a magnetic fluid, an optical reflective fluid, an optical transmissible fluid sealed in its slit are moved by using a magnetic field generator provided near the slit, similar to the above mechanical switch (Yasuhide Nishide et al., xe2x80x9cWaveguide Type Optical Switch,xe2x80x9d Japanese Patent Application No. 5-8854 (1993) [Official Gazette of Japanese Patent Application Laid-open No. 6-222294 (1994)]). Furthermore another optical switch similar to a mechanical switch has been proposed, which can switch the light path because refractive index matching liquid sealed in its slit is moved by heating micro thin-film heaters provided near the slit, similar to the above one (Hiroyoshi Togo et al. xe2x80x9cOptical Switch and Method of Fabricating the Switch,xe2x80x9d Japanese Patent Application Laid-open No. 10-333062 (1998]). An optical switch using a number of such kinds of optical switches has also been proposed.
The conventional mechanical switches, however, switch the light path by utilizing the reflection wall on only one side of the slit fabricated at the crossing point of the crossing optical waveguides.
FIGS. 5A and 5B show an example of an optical selector switch for a light path switching comprised of a conventional mechanical switch. In FIG. 5A, reference numeral 1 designates crossing optical waveguides, reference numeral 2 designates a slit which crosses in a diagonal direction at each crossing point of the optical waveguides 1, and reference numeral 3 designates refractive index matching liquid sealed in the slit 2 that has a refractive index equal to that of cores of the optical waveguides 1.
Specifically, in the normal (transmitting) state shown in FIG. 5A, when the reflective index matching liquid 3 is present at the crossing point of the optical waveguides 1, optical incident signals from an input end a of the crossing optical waveguide (in a horizontal direction of the drawing) pass straight through the slit 2 and are emitted to an output end A, as shown by the broken line. However, in the switching (reflecting) state in FIG. 5B, when the refractive index matching liquid 3 moves away from the crossing point of the optical waveguides 1, optical incident signals from the input end a are reflected by the total internal reflection on one side wall of the slit 2 near the input end a, and then emitted to an output end B as shown as a dot-line of the drawing.
In addition, as shown in FIG. 5A, when the refractive index matching liquid 3 is present at the crossing point of the optical waveguides 1, optical incident signals from an input end b of the crossing optical waveguide pass straight through the slit 2 and are emitted to the output end B. However, as shown in FIG. 5B, when the refractive index matching liquid 3 moves away from the crossing point of the optical waveguides 1, optical incident signals from the input end b cannot be totally reflected and propagated to the output end A because the side wall of the slit 2 near the input end b side is offset from an ideal reflection plane (a vertical plane located on a bisector of an interior angle at the crossing point of the optical waveguides 1) toward the input end b by a distance corresponding to the width of the slit 2. Thus, in this case, due to a decrease in an amount of the light propagated from the input end b to the output end A, the difference between the intensity of the optical signal outputted from the output end A and the intensity of the optical signal outputted from the output end B increases.
To eliminate the difference in optical signal intensity between the output ends A and B, an additional system for amplifying or attenuating the optical signal reflected by or passed through the slit is required, so that the entire structure of the optical switch becomes substantially complicated. This is disadvantageous in that fabrication cost of the optical switch increases and in that the price of the optical switch itself rises.
Thus, a single conventional mechanical switch provides only two inputs and two outputs (two bar functions) in the transmitting state, provides one input and one output (one cross function) in the reflecting state, and cannot work as 2xc3x972 switch having two bar functions and two cross functions.
Accordingly, to construct a 2xc3x972 optical switch using conventional mechanical switch elements, three or more mechanical switch elements, must be combined together in a form as shown in FIGS. 6A and 6B. FIG. 6A shows the normal (transmitting) state, while FIG. 6B shows the switching (reflecting) state. Moreover, to construct an optical array or a matrix switch using N combined 2xc3x972 optical switches, 3N or more of the above 2xc3x972 optical switches must be combined together. Consequently, to construct a large-scale optical matrix switch, a large number of switches are conventionally required, preventing size reduction.
As described above, the conventional optical switches are disadvantageous in that a 2xc3x972 optical switch cannot be constructed using a single optical switch and in that the size of a large-scale optical matrix switch cannot be reduced.
The present invention is created in view of the above problems of the prior art. It is a first object thereof to provide a mechanical switch which is capable of a cross/bar function of a 2xc3x972 optical switch through a single switch element, and which reduces a size of a large-scale optical matrix/array switch.
Furthermore, optical signals having different polarized lights depending on different wavelengths arrive at an optical node on this WDM type network. In this case, if the polarization dependence exists in a waveguide type optical part installed on the node, the optical signals have a varying intensity depending on each wavelength, the varying intensity may cause a primary factor degrading the flexibility and reliability of the entire network. Thus, the waveguide-type optical part arranged on the node must be insensitive to polarization.
However, the sufficient examination has not been done for the polarization dependence in relation to a waveguide type optical part including an mxc3x97n optical waveguide having a first group of plural (m) optical waveguides each having a parallel optical axis and a second group of plural (n) optical waveguides each having a parallel optical axis and crossing the first group of optical waveguides, and having reflective structure which can switch optical signals from the first group of optical waveguides to the second group of optical waveguides.
For the above waveguide-type optical parts with the reflective structure, we calculated, taking the Goos-Hxc3xa4nchen shift effect in the reflecting state into account, the distance of light soaked into a physical reflection plane (side wall) of the reflective structure: a phenomenon in which the light soaks into the physical reflection plane by a certain distance. Then, we have found that the distance by which the light soaks varies by the direction of a polarized incident light, that is, the polarization dependence exists.
Based on this discovery, it is a second object of the present invention to provide an optical part, particularly, an optical switch that can reduce the polarization dependence causing by a difference in the distance of light soaked which varies with the direction of the polarized light.
To attain the above first object, in an optical switch according to a first aspect of the present invention, a center line of a slit having two opposed side walls (physical reflection planes) is aligned with a bisector (ideal reflection plane) of an interior angle at a crossing point of optical-axis center lines of crossing optical waveguides, and a Goos-Hxc3xa4nchen shift effect on a light reflection plane (optical reflection plane) caused by light soaking is positively used to reduce an amount of positional deviation of the optical reflection plane from the ideal reflection plane. Thus, this optical switch utilizes both opposed side walls of the slit fabricated at the crossing point of optical waveguides crossing each other in an optical waveguide layer as the physical reflection plane that freely switches a light path of the crossing optical waveguides by actively utilizing the Goos-Hxc3xa4nchen shift effect. The optical switch can reduce the positional deviation of the reflection plane, and thereby simultaneously can switch the light paths in the crossing optical waveguides with a low loss. In addition, since the slit of the optical switch is located at the center of the crossing point of the crossing optical waveguides, and the optical switch uses both side walls of the slit as reflecting wall surfaces of the same characteristics, lights reflected by these side walls of the slit have an equal intensity.
Therefore, according to the first aspect of the present invention, this optical switch can provide a 2xc3x972 optical switch function through a single element without the needs for any amplifier or attenuator for equalizing the light intensity. Accordingly, the size of the 2xc3x972 optical switch, which conventionally requires three or more optical switches, can be reduced to a third part or less, thereby enabling this optical switch to be implemented at a low price. Further, an optical switch providing the 2xc3x972 switch function can be constructed using optical switch elements with a third part of elements that is required for the conventional optical switch.
In addition, according to a variation of the first aspect of the present invention, the above slit is disposed at all or some of the crossing points between a number of optical waveguides not crossing one another and a number of other optical waveguides not crossing one another in the optical waveguide layer so that the light path can be switched and routed between the crossing optical waveguides. Consequently, the size and number of the required switches can be reduced to by one-third part or less compared to the prior art, and the size of a large-scale optical array switch and a large-scale optical matrix switch can also be diminished.
In addition, compared to the conventional 2xc3x972 optical switch, which requires three or more optical switch elements, according to the present invention, the frequency of reflections required to switch light path decreases by one-third part or less, hence reflection loss caused by light path switching can be lessened by one-third part or less. Therefore, when an optical array switch or an optical matrix switch is constructed by using the optical switch according to the present invention, the total reflection loss of the optical switch can be reduced by one-third part or less.
As described above, the use of the optical switch according to the first aspect of the present invention enables the realization of a miniaturized 2xc3x972 optical switch and large-scale optical array/matrix switches with a low loss.
To attain the above second object, an optical switch according to a second aspect of the present invention consists of a crossing optical waveguide with reflective structures in mxc3x97n optical waveguides that are configured like a grid and include a first group of (m) optical waveguides each having a parallel optical axis and a second group of (n) optical waveguides each having a parallel optical axis and crossing the first group of optical waveguides individually, the reflective structures (ex. Slits) can switch optical signals from the first group of optical waveguides. The reflective structure""s interior is filled with air in the reflection state, cores of the first and second groups of waveguides have a refractive index equivalent to that of glass, an intersecting angle between the first and second group of optical waveguides is set based on the predetermined equation described below and is specifically limited between 0 and less than 90 degrees. This configuration can reduce the difference of the distance of light soaking which varies according to the polarizing direction and which makes the polarization dependent of the reflection loss. This configuration can provide a waveguide type optical part, particularly, an optical switch having a polorization insensitive that is negligibly level for an optical communication wavelength bandwidth 1.3 to 1.65 xcexcm.
However, the intersecting angle between the first and second group of optical waveguides is set to be low angle, optical signals from the first (second) group of optical waveguides are diffracted to the second (first) group of optical waveguides, resulting in a debased extinction ratio. Therefore, the intersecting angle between the first and second group of optical waveguides should be possibly taken a maxim degree within the above range. Preferably, in the waveguide type optical part, particularly, the optical switch, by limiting the intersecting angle between the first and second group of optical waveguides to 73 to 74 degrees, the difference in the distance of light soaking which varies according to the polarizing direction and which makes the polarization dependence of reflection loss is lessened to a negligible level, thereby providing a waveguide type optical part, particularly, an optical switch has a polarization insensitive that is negligibly level for an optical communication wavelength bandwidth 1.3 to 1.65 xcexcm.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.